Fluorescent lamp dimming circuit employing an improved auxiliary circuit

ABSTRACT

An adjustable dimming circuit is provided to control light output levels of a fluorescent lamp. The dimming circuit comprises an intensity selector circuit controlling an auxiliary circuit, and a ballast circuit which is controlled by the auxiliary circuit. The intensity selector circuit provides timing pulses to control the auxiliary circuit. The auxiliary circuit comprises a control switch and a power switch. The control switch is gated on by the intensity selector circuit and controls the operation of the power switch by providing gate current for the power switch after the control switch comes on until zero crossover current in the control switch. The ballast circuit is used to limit current through the fluorescent lamp and to provide starting pulses for the lamp. The power switch in the auxiliary circuit controls the timing of the application of the starting pulses applied to the lamp, and also provides a current path during the operation of the lamp.

United States Patent [1 1 I-Ierzog Jan. 28, 1975 1 FLUORESCENT LAMP DIMMING CIRCUIT EMPLOYING AN IMPROVED AUXILIARY CIRCUIT [75] Inventor: Rollie R. Herzog, Danville, Ill.

[73] Assignee: General Electric Company, New

York, NY.

[22] Filed: Apr. 6, I973 [21] Appl. No.: 348,676

INTENSITY Primary Examiner-William L. Sikes Assistant Examiner-Richard A. Rosenberger [57] ABSTRACT An adjustable dimming circuit is provided to control light output levels of a fluorescent lamp. The dimming circuit comprises an intensity selector circuit controlling an auxiliary circuit, and a ballast circuit which is controlled by the auxiliary circuit. The intensity selector circuit provides timing pulses to controlthe auxiliary circuit. The auxiliary circuit comprises a control switch and a power switch. The control switch is gated on by the intensity selector circuit and controls the operation of the power switch by providing gate current for the power switch after the control switch comes on until zero crossover current in the control switch. The ballast circuit is used to limit current through the fluorescent lamp and to provide starting pulses for the lamp. The power switch in the auxiliary circuit controls the timing of the application of the starting pulses applied to the lamp, and also provides a current path during the operation-of the lamp.

11 Claims, 2 Drawing Figures SELECTOR Q '13 BALLAST CIRCUIT J PATENTED JAN28 I975 sneer 2 0F 2 IN PUT VOLTAGE /LAMP CURRENT FLUORESCENT LAMP DIMMING CIRCUIT EMPLOYING AN IMPROVED AUXILIARY CIRCUIT BACKGROUND OF THE INVENTION This invention relates to an improved dimming circuit for fluorescent lamps, and more particularly to an improved auxiliary circuit used in a dimming system which is useful at all light levels.

In general, dimming circuits for fluorescent lamps utilize an auxiliary circuit in the form of power switch which is controlled by an intensity selector. The power switch, whichis usually a thyristor, supplies current to the lamps at various illumination or dimming levels by controlling the interval of current conduction through lamps in each half cycle of the power supply. The intensity selector which controls the thyristor is made variable to provide continuous adjustment of illumination.

One of the major problems in the prior art dimming system has been flicker. Flicker is an instability or variability of light level and is very annoying to the viewer. Flicker may occur at the higher or lower light levels.

Flicker may occur at high intensity light levels because the lamp is turned on during one half cycle of possible conduction much earlier or later than the next half cycle of possible conduction. As a. result, the amount of light produced may vary from one half cycle to the next. This may be better understood by referring to FIG. 2 which shows waveforms of input voltage, lamp current, and timing pulses for a prior art dimming system operating near full intensity. A timing pulse gates the thyristor on at point a in the lamp current half cycle and the thyristor may latch on until the lamp current goes to zero. It is desired that a timing pulse gate on the thyristor as early in the negative half cycle as occurred in the positive half cycle, e.g., point a in order to operate at near full intensity. However, because the timing circuit operates from leading input voltage the timing pulses did not occur at identical places on the half cycles. FIG. 2 shows a timing pulse occurring at point e which was meant to occur after the beginning of the negative current half cycle. This pulse will not effectively turn on the thyristor in the negative half cycle because as soon as the current goes through zero the thyristor will turn off. It will not come on again until another timing pulse starts it at point d. The lamp will, therefore conduct more in the positive half cycle and less in the negative half cycle resulting in flicker. This type of flicker doesnt .usually occur at low intensity levels because the timing pulses are not applied near zero current crossover.

Low intensity flicker occurred because the timing circuit was unregulated. Variation in components, such as a unijunction transistor, and line voltage fluctuation caused the flicker to occur at low intensity light levels.

These types of flicker are somewhat overcome in the prior art by the use of high and low trim potentiometers in conjunction with a master potentiometer, all of which affect the frequency of the oscillator. The trim potentiometers could be used to adjust out the flicker by firing the thyristor after zero lamp current. This method is often misunderstood especially in the initial adjustment of the trim potentiometers.

Another problem with prior art dimming circuits has been that the thyristor or auxiliary circuit required the use of a high wattage current holding resistor to maintain conduction at low dimming levels. This resistor usually was connected in series with the thyristor and was placed in the ballast circuit where it dissipated as much as 5 watts. This resistor caused a power loss and substantial heating in the ballast, both of which are undesirable.

In prior art dimming systems which used a control switch to control the power switch, another high wattage resistor was used to drop some of the line voltage so that full line voltage was not across the control switch. This also caused heating in the dimming system.

It is desirable, therefore, to provide a circuit capable of dimming fluorescent lamps which does not require trim potentiometers to overcome flicker. It is further desirable to eliminate the need for a high wattage current holding resistor and the need for a high wattage resistor to drop the voltage for the control switch.

SUMMARY OF THE INVENTION Accordingly, it is a general object of this invention to provide an improved dimming circuit for fluorescent lamps.

Another object of the invention is to provide an improved dimming circuit which eliminates flicker in fluorescent lamps at all light levels.

Another object of the invention is to provide such a dimming circuit which eliminates the need for a high wattage current holding resistor for maintaining conduction of the power switch in the auxiliary circuit.

Another object is to eliminate the need for a high wattage voltage dropping resistor for operating a control switch.

Still another object of the invention is to provide a dimming circuit which eliminates the need for trim potentiometers in the intensity selector circuit.

In accordance with one form of this invention there is provided a gaseous discharge lamp dimming circuit including a bilateral current conducting control switch having a control electrode and a step down transformer connected to the control switch and a source of A.C. voltage. A gating signal is provided to turn on the control switch and the control switch will conduct until its current goes to zero crossover. A bilateral current conducting power switch having its own control electrode is further provided. The power switch control electrode is connected to the control switch and is continuously gated by the control switch even when the current in the power switch falls below the holding current level.

Because the power switch is continually gated to its on condition, even if the lamp current goes through zero, the need for a high trim potentiometer in series with the master potentiometer in the intensity selector circuit and the need for a high wattage holding resistor for the power switch are elminated. Furthermore, the need for a high wattage voltage dropping resistor, which connected the control switch to the A.C. source, is also eliminated by the inclusion of the step down transformer.

BRIEF DESCRIPTION OF THE DRAWINGS The subject matter which is regarded as the invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may be better understood by referring to the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic circuit diagram of one embodiment of the dimming circuit including an intensity selector circuit, an auxiliary circuit, and a ballast circuit.

FIG. 2 is a diagram of the waveforms of a prior art dimming system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the dimming circuit comprises an intensity selector circuit 11, an auxiliary circuit 12 and a ballast circuit 13. The intensity selector circuit controls the auxiliary circuit. The auxiliary circuit, about which this invention is primarily directed, controls the ballast. The ballast controls the starting of a gaseous discharge lamp which, in this embodiment, is a fluorescent lamp and it also controls the current through the lamp.

The intensity selector circuit 11 utilizes a timing circuit to initiate lamp dimming. This timing circuit includes a programmable unijunction transistor or PUT Q, having anode, cathode, and gate electrodes. A capacitor C, is connected to the anode of PUT Q, and provides anode-to-cathode current to PUT Q,. A charging path for capacitor C, is provided by a series circuit comprising resistors R, and R and diodes D, and D A transformer T, is connected to diodes D, and D and to an A.C. power supply to provide power for the intensity selector circuit. To provide voltage regulation for the intensity selector circuit, a Zener diode D is connected across a series circuit of R and PUT Q,. Resistors R R and R,, form a voltage divider. The gate of the PUT is connected to the junction of the divider between resistor R ano parallel resistors R and R,,. The parallel circuit includes variable resistor R,, and is known as the master potentiometer and controls the frequency of the timing circuit. The frequency of the timing circuit ultimately controls the dimming level of lamp L. To this end an SCR Q which has its gate electrode connected to the cathode of PUT 0,, forms the output of the intensity selector circuit is connected to the auxiliary circuit 12.

The illustrative intensity selector circuit 11, as described above, is somewhat simplified for the sake of ease of understanding. A more detailed description of various embodiments of an intensity selector circuit useful in the present dimming circuit is provided in US. Pat. No. 3,767,940, issued Oct. 23, 1973 and assigned to the General Electric Company, assignee of the present invention.

The auxiliary circuit 12 is connected to the output of the intensity selector circuit. The auxiliary circuit includes a series circuit consisting or resitor R capacitor C and primary winding 16 of transformer T Resistor R and winding 16 provide a charge path for capacitor C, from Zener diode D Primary winding 16 also provides a discharge path for capacitor C when SCR O2 is turned on. The secondary winding of transformer T is connected to bilateral current conducting control switch 14. Resistor R is connected across a secondary winding 15 to eliminate false gating of the control switch. The control switch comprises a diode bridge D and an SCR Q which is connected across the output of the diode bridge. This switch arrangement provides full wave alternating current through itself.

Power for the control switch is obtained from step down transformer T having a primary winding 17 and a secondary winding 18. In some prior art circuits a high wattage voltage dropping resistor was used to supply power to the control switch at a voltage which was near or below the rating of the control switch. By providing a step down transformer, this resistor is no longer necessary.

The control switch 14 acts as a latch. That is. once it is gated on by the voltage induced by secondary 15, it stays on until its zero current crossover. The output of the control switch is connected to the gate electrode of power triac Q Capacitor C and resistor R are connected across power triac Q, in order to overcome dv/dt which may cause false triggering of the power triac. The power triac serves to switch current through the ballast circuit 13 and the fluorescent lamp L. Since the control switch has its output connected to the gate of power triac Q, and serves as a latch. there is continual gating current for triac Q, while control switch l4 conducts. Even if the auxiliary 12 is pulsed by the timing circuit before zero lamp current, triac Q will stay on after zero lamp current because of this continual gating. Furthermore, this continual gating holds triac Q, on even at low lamp current levels. This obviates the need for a high wattage current holding resistor for the power triac during low intensity levels and furthermore, obviates the need for a high trim potentiometer formerly required in the intensity circuit during operation at high intensity levels.

The ballast circuit includes primary coils l9 and 20 of transformer T coils 19 and 20 receive power from a 277 volt supply. When the ballast is operating at volts, the coil 19 alone is used as a primary. Capacitor C and secondary coil 22 form a peaking circuit which provides a voltage spike to turn on lamp L. Resistor R,, is connected to secondary coil 22 and provides a charge path for capacitor C The peaking circuit is further connected to power triac Q, in the auxiliary circuit. Coil 22 and power triac Q provide a discharge path for capacitor C when power triac 0, comes on. Secondary coils 24 and 25 are magnetically coupled to the primary coils 19, 20 and provide heating for the cathodes of the fluorescent lamp. Capacitor C, is connected across primary 19, 20 to provide power factor correction. Resistor R,,, which is a low wattage resistor dissipating one-half watt maximum, is connected in series with power triac O to provide stabilization during the discharge time of capacitor C in the prior art a high wattage current holding resistor was used in place of resistor R,,. This caused a power loss of up to five watts and heating in the ballast. By continuously gating the power triac, the need for this high wattage resistor is no longer necessary.

A more detailed description of the ballast circuit is provided in Application Ser. No. 348,67l filed Apr. 4. 1973 and assigned to the General Electric Company.

The overall operation of the dimming circuit is as follows:

Resistor R and variable resistor R,,, known as the master potentiometer are adjusted to provide the light level or dimming level desired. Capacitor C,, having charged through resistors R, and R is discharged through PUT Q, when the PUT anode voltage exceeds its gate voltage. This turns on SCR Q Capacitor C, is charged through diode D, and resistor R, and is discharged through primary winding 16 whe SCR. 0, comes on. This causes a pulse to be supplied to secon dary winding 15, turning on SCR 0,, which forms a part of controlswitch 14. A full wave current path to the control switch 14 is established through resistor R secondary winding 18, and the gate-anode of TRlAC Q Control switch 14 latches on until zero current through the control switch occurs. Control switch 14 gates power tricac Q, to its on condition. The gating on of the triac is independent of the lamp current which lags the supply voltage. Capacitor C having been charged through resistor R and secondary coil 22 is discharged through secondary coil 22 and power triac 0., when power triac comes on. This provides a peaking voltage to turn on the lamp L by inducing high voltage from secondary coil 22 into secondary coils 21 and 23. A current path is established through the primary coil 19, secondary coil 23, through lamp L through secondary coil 21 and power triac Q The light level of lamp L may be varied by adjusting the master potentiometer R In prior dimming circuits it was necessary to use a high trim potentiometer in series with the master potentiometer in the intensity selector circuit in order to prevent flicker at the high intensity output levels. Flicker occurred because the power triac was turned on before zero lamp current. A high trim potentiometer was used to change the timing and thus fire the triac after zero lamp current. By using applicants method of continuously gating the triac, the necessity of the high trim potentiometer at high intensity levels is obviated. Another result of ocntinuously gating the power triac is to overcome the need of the high wattage current holding resistor in series with the triac. The high wattage, voltage dropping resistor for the control switch has also been eliminated by the step down transformer.

The circuit as set forth in FIG. 1 has been built and operated with components having the following values:

Resistor R, l K R 100 K R 1 K R 15 K R K R, 100 K R 100 ohms R 220 K R, 1 x R, 2 Meg R 10 K pot R 100 ohms R 680 ohms R 4.7K Diode D 400 V, 1 A D 400 V, l A D 10 V Zener, 100 MA 0, 400 V, l A D Full Wave Bridge with 4 Diodes 400 V DC, 1 A DC, 280 V RMS PUT Q 2N6027 SCR 0: 2N4184 Q, C106B2 6.15. No. TRlAC Q, 500 V, amp 85C Transformer T,

primary winding 1690 turns, .0063 I11. dia. secondary winding-2 windings (420 turns, .0063 in. dia.)

Transformer T primary winding 17 1690 turns, .0063 m. dia. secondary winding 18 132 turns, .010 in. dia.

Transformer T I primary winding 15 1000 turns, .0045 m (118 secondary winding 16 1000 turns .0045 m. dla.

Transformer T primary winding 19 1048 turns, .0071 [11. dia. primary winding 20 1241 turns, .010 111 1113. secondary winding 21 598 turns, .01 l9 m. dla. secondary winding 22 358 turns .0071 In. dra. secondary winding 23 373 turns, .01 19 in. dia. heating winding 24 42 turns, .010 in dia. heating winding 25 42 turns, .010 In dra.

Capacitor C .047 uf C .01 uf C .047 uf C .05 uf C .01 uf C 1.9 uf C .01 uf C .l5 uf From the foregoing description of the embodiment of the invention, it-will be apparent that many modifications may be made therein. It will be understood that this embodiment of the invention is intended as an exemplification of the invention only, and that the invention is not limited thereto. For example, in the auxiliary circuit shown, control switch 14 comprises an SCR connected across a full wave diode bridge. Other full wave latching devices may be employed as a control switch, such as back-to-back SCRs or a triac. Also in the auxiliary circuit, a triac is disclosed as the power switch. Other power switches, such as a bilateral transistor, may also be employed. It is to be understood, therefore, that it is intended in the appended claims to cover all such modifications which fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A gaseous discharge lamp dimming circuit for connection to a source of AC voltage comprising:

a bilateral current conducting control switch having a control electrode;

means for connecting said control switch to the source of AC voltage;

means including a timing means, for applying a signal to said control electrode to render said control switch conductive, said control switch conducting current thereafter until the control switch current goes through zero crossover;

a bilateral current conducting power switch having a control electrode;

said control switch being connected to said control electrode of said power switch for continuously gating said power switch during conduction of said control switch so that said power switch remains on even if the current through said power switch is below the normal power switch holding current level;

a ballast circuit connected to said power switch for limiting the lamp current.

2. The dimming circuit as set forth in claim 1, wherein said power switch includes at least one thyristor.

3. The dimming circuit as set forth in claim 1, wherein said control switch comprises a full-wave rectifier ridge and a thyristor connected across said bridge.

4. The dimming circuit as set forth in claim 1, further including an isolation transformer for coupling said control electrode of said control switch means to said timing means.

5. The dimming circuit as set forth in claim 1, further including a step down transformer for connecting said control switch to the source of AC. voltage.

6. The dimming circuit as set forth in claim 1, wherein said power switch is a Triac.

7. A gaseous discharge lamp dimming circuit comprising:

a bilateral current'conducting control switch having a control electrode; step down transformer connected to said control switch and adapted to be connected to a source of AC. voltage for application of a relatively low voltage to said control switch; means for applying a signal to said control electrode of said control switch, said control switch conducting current thereafter until the control switch current goes through zero crossover; a bilateral current conducting power switch having a control electrode; said control switch being connected to said control electrode of said power 7 8 switch for providing continuous gate current for 10. The dimming circuit as set forth in claim 7 said power switch during the conduction of said wherein said control switch includes at least one thycontrol switch. ristor. 8. The dimming circuit as set forth in claim 7 wherein 11. The dimming circuit as set forth in claim 9 said power switch includes at least one thyristor. wherein said control switch includes a full wave bridge 9. The dimming circuit as set forth in claim 8 wherein and an SCR connected across said bridge.

said at least one thyristor is a Triac. 

1. A gaseous discharge lamp dimming circuit for connection to a source of AC voltage comprising: a bilateral current conducting control switch having a control electrode; means for connecting said control switch to the source of AC voltage; means including a timing means, for applying a signal to said control electrode to render said control switch conductive, said control switch conducting current thereafter until the control switch current goes through zero crossover; a bilateral current conducting power switch having a control electrode; said control switch being connected to said control electrode of said power switch for continuously gating said power switch during conduction of said control switch so that said power switch remains on even if the current through said power switch is below the normal power switch holding current level; a ballast circuit connected to said power switch for limiting the lamp current.
 2. The dimming circuit as set forth in claim 1, wherein said power switch includes at least one thyristor.
 3. The dimming circuit as set forth in claim 1, wherein said control switch comprises a full-wave rectifier ridge and a thyristor connected across said bridge.
 4. The dimming circuit as set forth in claim 1, further including an isolation transformer for coupling said control electrode of said control switch means to said timing means.
 5. The dimming circuit as set forth in claim 1, further including a step down transformer for connecting said control switch to the source of A.C. voltage.
 6. The dimming circuit as set forth in claim 1, wherein said power switch is a Triac.
 7. A gaseous discharge lamp dimming circuit comprising: a bilateral current conducting control switch having a control electrode; a step down transformer connected to said control switch and adapted to be connected to a source of A.C. voltage for application of a relatively low voltage to said control switch; means for applying a signal to said control electrode of said control switch, said control switch conducting current thereafter until the control switch current goes through zero crossover; a bilateral current conducting power switch having a control electrode; said control switch being connected to said control electrode of said power switch for providing continuous gate current for said power switch during the conduction of said control switch.
 8. The dimming circuit as set forth in claim 7 wherein said power switch includes at least one thyristor.
 9. The dimming circuit as set forth in claim 8 wherein said at least one thyristor is a Triac.
 10. The dimming circuit as set forth in claim 7 wherein said control switch includes at least one thyristor.
 11. The dimming circuit as set forth in claim 9 wherein said control switch includes a full wave bridge and an SCR connected across said bridge. 